In recent years, digital still cameras (hereinafter referred to simply as digital cameras) that enable input of picture image information, such as photographed landscapes and portraits, into a personal computer have rapidly become popular with the popularity of personal computers in households. In addition, along with advanced functions of cellular phones, camera phones that incorporate compact image pickup modules have also rapidly become popular. Furthermore, including an image pickup module in compact information terminal equipment, such as PDAs, has also become popular.
In such devices that include an image pickup function, an image pickup element, such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor), is used to provide the imaging function, and in recent years the size of such image pickup elements has rapidly decreased. Therefore, image pickup devices using such image pickup elements now need to be small and lightweight. Additionally, image pickup elements with a larger number of pixels in the same area have been developed in order to achieve higher image quality, which requires higher resolution lens systems that are still very compact and yet provide excellent imaging performance.
Japanese Laid-Open Patent Application H10-048516 describes exemplary single focus lenses for such devices that include only three lens components, each of which is a lens element, with the image-side lens element having a biconvex shape and being immediately preceded on its object side by an aperture stop.
As described above, recent image pickup elements are smaller and provide more pixels in a given detector area, which helps meet demands of higher resolution and more compactness that are especially required in imaging devices for digital cameras. On the other hand, the primary requirements for image pickup lenses of a small information terminal device, such as a camera phone, are conventionally low cost and compactness. However, recent camera phones tend to have an image pickup element with a larger number of pixels. For example, image pickup elements with megapixel detectors (detectors that detect one million or more pixels) are being used practically. High performance is also increasingly demanded. It is desirable to develop a variety of lenses to meet a variety of user needs considering all the factors together, including cost, performance, and compactness. For example, it is desirable to develop a low cost and high performance image pickup lens that is compact for mounting in a portable modular camera and, based on its high performance, also in a digital camera.
In order to meet these requirements, lenses with three or four lens components, each of which components may be a lens element, have been considered in order to achieve low cost and compactness and to use aspheric surfaces for high performance. However, although aspheric surfaces assist in obtaining compactness and higher performance, they are disadvantageous in terms of productivity and cost. Therefore, aspheric surfaces should be used with careful consideration of productivity. The single focus lenses described in Japanese Laid-Open Patent Application H10-048516 discussed above use three lens components that are lens elements and include aspheric surfaces, but the comprehensive performance of these lenses is unsatisfactory. If the optical performance is made adequate for various purposes, the lenses are not sufficiently compact. Generally, a lens with only three lens elements is sufficient in performance for a portable modular camera, but the performance is insufficient for a digital camera.